1. Field of the Invention
The present invention relates to fabrication of semiconductor device capacitor, and more particularity to a method for forming an electrode of semiconductor device capacitor.
2. Description of the Prior Art
Recently, the demand of semiconductor devices has increased rapidly owing to widespread use of electronic equipment. In particular, the increasing popularity of some electronic equipment is increasing the demand for minimized semiconductor devices. By the way, to minimize elements of semiconductor devices is a contemporary tendency for producing integrated semiconductor devices.
As the semiconductor device is highly integrated, there are some problems happened. For example, the area occupied by a capacitor is shrank and the capacitance of the capacitor is decreased owing to its smaller electrode surface. No matter how, a plurality of specific semiconductor devices require a relatively large capacitance for reliable operation. For example, the memory cell requires high capacitance to achieve a high signal-to-noise ration. Therefore, it is desirable to reduce the capacitor scale for device integration and yet obtains enough capacitance for operation of capacitor.
One approach for solving the problem is toward the shape of the capacitor electrode, which is usually a conductive structure, for example a polysilicon rod. The essential point is increasing the total surface of the capacitor electrode when the occupied area of capacitor electrode on the semiconductor device is small. One important method is stacked design, with the technology two or more layers of conductive layers such as polycrystalline silicon (polysilicon) are deposited on a semiconductor substrate, with dielectric layers sandwiched between each conductive layer. In additional, the capacitance can be enhanced by using the hemi-spherical grain (HSG) silicon on an electrode to form a rugged surface but without increasing the area required for the electrode. It means that a polysilicon rod is applied to form the basic structure of electrode and HSG silicon is formed on surface of the polysilicon rod to increase total surface of electrode.
Because the growth of HSG demands amorphized silicon, the polysilicon rod must be amorphized. Replace polysilicon rod totally by amorphized silicon rod is not an efficient way, owing to the growth time of amorphized silicon is long and then the probability of crystallization of silicon is not negligible. However, the growth of HSG silicon only reacts with the surface part of electrode so only the surface part of polysilicon rod needs to be amorphized. Two processes are usually used to form the amorphized silicon, one is the implantation process and another is the spacer process. The former uses impurity particles to implant the surface of polysilicon electrode and transforms the structure of surface part from polysilicon into amorphized silicon. The process is an efficient process. But when a plurality of electrodes is formed by the process, an obvious disadvantage happens. If the distance between any two neighboring electrodes is short or the height of electrode is large, these neighboring sidewalls of electrodes are screened by other electrodes and these implanting particles are impeded. The result is that the thickness of amorphized silicon is not uniform, that is, it is thicker in top part of sidewall but is thinner or even zero in bottom part of sidewall. By the way, the growth of HSG silicon in the bottom part of sidewall of electrode is restricted by the thickness of amorphized silicon. FIG. 1A shows the effect, where the thickness of amorphized silicon 10 is non-uniform in neighboring sidewalls of two polysilicon rods 12, but is uniform in other sidewalls. Another process for growth of HSG silicon is the spacer process, it includes two steps as shows in FIG. 1B and FIG. 1C. First amorphized silicon layer 14 is formed on top of polysilicon rods 12 and then amorphized silicon spacers 16 are formed in sidewalls of polysilicon is rods 12. By the process, the thickness of amorphized silicon is always enough in every part of sidewall for growth of HSG silicon, but the process consumes much time for two deposition processes and some photolithography processes are necessary and then throughout of the process is slow.
Thus, there is a need for an efficient method to form amorphized silicon on surface of polysilicon rod with enough thickness everywhere, such that hemispherical grain silicon is growing on total surface of polysilicon rod and then the capacitance of capacitor is increased.